Dynamical effects as a window into van der Waals interactions in grazing-incidence fast He-atom diffraction from KCl(001)

Abstract

In this work we address, both experimentally and theoretically, the very grazing scattering of He atoms off KCl(001) with incidence along the 100 channel. Our theoretical model combines a semiquantum description of the scattering dynamics and a high-precision interaction potential. By means of a thorough analysis of the quantum phase for in-plane scattering and rainbow trajectories, we are able to connect the presence of the physisorption well with the significant enhancements of the corrugation and rainbow angle, relative to the hard corrugated wall predictions. We trace this connection to dynamical effects on the incident and scattered beams due to their traversing of the physisorption well. Finally, we show that the inclusion of vdW interactions in the potential improves the theoretical accord with experiments for both the corrugation and the rainbow angle.

Temperature dependence in fast-atom diffraction at surfaces

“…This naive, purely repulsive description was improved by taking into account an attractive part of the potential, for instance, van der Waals contributions, [44][45][46][47] responsible for the physisorption well of depth D. In elastic diffraction, the effect of such attraction is the presence of bound-state resonances 48,49 and the increase of the rainbow angle at low energy. 22 These are naturally accounted for using a quantum approach 22,49 or modeled in semi-classical 50 or optical method such as the hard corrugated wall by the Beeby correction indicating that the effective impact energy E > increases to E > + D 44,51,52 or considering a modified angle of incidence y eff ¼ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi y in 2 þ D=E p . The Beeby correction also decreases significantly the DWF in TEAS.…”

Perturbation theory of scattering for grazing-incidence fast-atom diffraction

“…In the present work we aim to reproduce the essential features of recent experiments and numerical calculations of the potential. 10,11 Specifically, we adopt an approach that corrugates the depth of the adsorption well by allowing the amplitude of the displacement, h(x), to be different for the attractive and the repulsive terms in eqn (2.1). Thus,…”

“…In recent work on He scattering from a KCl(001) surface, 10,11 the theoretical analysis of experimental data and the implications for classical scattering indicated an ''unexpected'' energy dependence in that both the corrugation of an ''effective hardwall'' potential, and the corresponding rainbow angle increased as the experiment came closer to grazing incidence. As we show below, the energy dependence of the rainbow angle is not unexpected theoretically 12 and has been observed experimentally in the scattering of Ar on a 2H-W(100) surface 13 and most notably in the case of scattering of Ar on a LiF(100) surface.…”

“…It may alternatively be calculated in the semi-classical picture from the action along classical trajectories, a quantity Bocan et al call the ''dynamic corrugation''. 10,11 These two definitions are not the same and to compare them directly is unhelpful. In the present work, we clarify the dynamics by discussing a simple model of the interaction, which allows the phenomenology in the scattering to be attributed directly to features of the potential.…”

“…In the present work, we clarify the dynamics by discussing a simple model of the interaction, which allows the phenomenology in the scattering to be attributed directly to features of the potential. The model is used to explain why the observed energy dependence of the classical rainbow angle in the scattering of He/KCl(001) 10,11 is not at all ''unexpected''. Not less important is our observation that the action-derived corrugation amplitude and the rainbow angle are, to first order in perturbation theory, proportional to each other and therefore provide the same information on particular aspects of the potential morphology.…”